6lo P. Thubert, Ed.
Internet-Draft Cisco
Intended status: Standards Track Z. Brodard
Expires: January 25, 2018 Ecole Polytechnique
H. Jiang
G. Texier
Telecom Bretagne
July 24, 2017
A 6loRH for BitStringsdraft-thubert-6lo-bier-dispatch-03
Abstract
This specification extends the 6LoWPAN Routing Header to signal
BitStrings such as utilized in Bit Index Explicit Replication and its
Traffic Engineering variant.
Status of This Memo
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This Internet-Draft will expire on January 25, 2018.
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Internet-Draft A 6loRH for BitStrings July 20172. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
The Terminology used in this document is consistent with and
incorporates that described in Terms Used in Routing for Low-Power
and Lossy Networks (LLNs). [RFC7102].
Other terms in use in LLNs are found in Terminology for Constrained-
Node Networks [RFC7228].
The term "byte" is used in its now customary sense as a synonym for
"octet".
"RPL", "RPL Packet Information" (RPI) and "RPL Instance" are defined
in the RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks
[RFC6550] specification.
The terms Bit-Forwarding Egress Routers (BFR), BFR-id and BitString
are defined in [I-D.ietf-bier-architecture]. A BitString indicates a
continuous sequence of bits indexed by an offset in the sequence.
The leftmost bit is bit 0 and corresponds to the value 0x80 of the
leftmost octet in the BitString.
3. Applicability
BIER and other bit-indexed methods that would leverage BitStrings
will generally require additional information in the packet to
complement the BitString. For instance, BIER has the concept of a
BFR-id and an Entropy value in the BIER header. Since those
additional fields depend on the bit-indexed method, they are expected
to be transported separately from the BitString. This specification
concentrates on the BitString alone.
Within the context of "the Deterministic Networking (DetNet)
Architecture" [I-D.ietf-detnet-architecture] ), the "BIER-TE-based
OAM, Replication and Elimination"
[I-D.thubert-bier-replication-elimination] document details how BIER-
TE can be leveraged to activate the Deterministic Networking
Replication and Elimination functions in a manner that is abstract to
the data plane forwarding information. An adjacency, which is
represented by a bit in the BIER header, can be mapped in the data
plane to an Ethernet hop, a Label Switched Path, or it may correspond
to a loose or a strict IPv6 Source Routed Path.
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Internet-Draft A 6loRH for BitStrings July 2017
In the context of LLNs, the 6TiSCH Architecture
[I-D.ietf-6tisch-architecture] introduces the concept of a Track that
is a directional traffic-engineered path between a source and a
destination. A Track is indicated in a packet by a Source or
Destination IPv6 Address and a RPL Local Instance. The RPL Instance
is carried in an IPv6 packet as part of the RPL Packet Information
(RPI), and a bit in the RPI indicates whether the Instance is Local
to the Source or the Destination Address. The RPI can be compressed
as a RPI 6LoRH header (RPI-6LoRH) as described in [RFC8138].
The 6TiSCH requirements for DetNet [I-D.thubert-6tisch-4detnet]
indicate that a 6TiSCH Track may leverage replication and elimination
as defined in DetNet. This specification enables this behavior as
follows: if a BIER-6LoRH is positioned right after a RPI-6LoRH, then
the BitString in the BIER-6LoRH applies to the context of the Track
indicated by the source or destination address of the packet and the
local Instance ID associated to the source or destination of the
packet.
4. The BIER-6LoRH encoding
The BIER 6LoRH (BIER-6LoRH) is a Critical 6LoWPAN Routing Header that
provides a variable-size container for a BitString such as, a but not
limited to, a BIER BitString.
The capability to parse the BIER BitString is necessary to forward
the packet so the Type cannot be ignored.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
|1|0|0| Control |6LoRHType 15-24| BitString |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+
Figure 1: The BIER-6LoRH
This specification provides a 5-bit Control field that can be used to
encode information that is specific to the BitString. The type and
size of the BitString are encoded in the 6LoRHType.
4.1. The Bit-by-bit BitStrings
In the bit-by-bit case, each bit is mapped in an unequivocal fashion
with a single addressable resource in the network. This may rapidly
lead to large BitStrings, and BIER allows to divide a network into
groups that partition the network so that a given BitString is
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Internet-Draft A 6loRH for BitStrings July 2017
locally significant to one group only. This specification uses the
5-bits Control field to encode the group.
When groups are used, it may be that a packet is sent to different
groups at the same time. In that case, multiple BIER-6LoRH headers
can be prepended to a same packet, each one for a different group.
As the packet flows along the multicast distribution tree, a BIER-
6LoRH header that has no more destination in a given branch may be
removed to make the packet shorter.
4.2. Bloom Filters
A Bloom Filter can be seen as a compression technique for the
BitString. A Bloom Filter may generate false positives, which, in
the case of BIER, result in undue forwarding of a packet down a path
where no listener exists.
As an example, the Constrained-Cast [I-D.bergmann-bier-ccast]
specification employs Bloom Filters as a compact representation of a
match or non-match for elements in a set that may be larger than the
number of bits in the BitString.
In the case of a Bloom Filter, a number of Hash functions must be run
to obtain a multi-bit signature of an encoded element. This
specification uses the 5-bits Control field to signal an Identifier
of the set of Hash functions being used to generate a certain
BitString, so as to enable the migration from a set of Hash functions
to the next.
4.3. Types of BIER-6LoRH header
The Type of a BIER-6LoRH header indicates the size of words used to
build the BitString and whether the BitString is operated as an
uncompressed bit-by-bit mapping, or as a Bloom filter.
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